The present invention relates to a novel thermal swing adsorption (TSA) process for the removal of hydrocarbons and oxides of nitrogen from air. More particularly, the present invention relates to a TSA process for use in pre-purification units (PPU) prior to cryogenic distillation of air to separate it into oxygen and nitrogen.
Prior to cryogenic air separation by distillation to produce oxygen and nitrogen, various trace impurities must be removed from feed air to avoid formation of solids in the heat exchanger equipment and resultant high pressure and safety issues in the cryogenic process. The most obvious trace air impurities that must be removed include carbon dioxide, CO2, and water, H2O. There are many references which disclose the use of pressure swing adsorption, PSA, and thermal swing adsorption, TSA, to remove these types of impurities from air in pre-purification units prior to cryogenic separation of air by its distillation into oxygen nitrogen and argon.
The importance of the removal of nitrogen oxides from air prior to entering into a cryogenic air separation plant has only recently been recognized. The removal of dinitrogen oxide, N2O , which is also known as nitrous oxide, is particularly important because of its increase in concentration in the atmosphere. It is well known that N2O is a greenhouse gas and the concentration of N2O in the atmosphere (currently about 0.3 ppm) has been increasing steadily (by about 0.2 to 0.3% annually), over the last decade. This increase is mainly caused by anthropogenic activities as well as by emissions from various chemical processes. An excess of N2O in cryogenic air separation units may lead to tube p contamination of the products. The fact that N2O is very stable in air, viz., its xe2x80x9clifetimexe2x80x9d in the atmosphere comprises about 150 years, makes the removal of N2O in an air pre-purification unit absolutely essential in both present time and the future. It is envisioned that in the future the removal of N2O will become as important as the removal of water and CO2. As the concentration of N2O in air increases further, the current regime of PPU processes will become inadequate because N2O cannot be removed easily by existing PPU processes. Accordingly, there is a clear need to develop an approved adsorption process suitable for use in PPU units to remove not only water and CO2 but also the trace amounts of nitrogen oxides, in particular N2O, which are present in the air being sent to th addition, great care should be taken for the removal of traces of hydrocarbons, such as low-molecular weight hydrocarbon gases ethane, propane, n-butane, iso-butane as well as any non-saturated species, such as acetylene, ethylene, propylene, the n-butylene isomers and iso-butylene, from air in PPU processes. It is also important that both hydrocarbons and plugging components such as N2O and CO2 be simultaneously removed in the air pre-purification process. Deposits of plugging components may create an opportunity for hydrocarbons to collect and concentrate in those deposits. The present invention is directed to such a solution, which aims, in particular, to a simultaneous, highly effective pre-purification of air.
N2O is not the only hazardous trace impurity present in atmospheric air. N2O belongs to a class of hazardous impurities collectively referred to as plugging/plating components. Water and CO2 are the other plugging/plating components removed in the PPU. These impurities freeze at cryogenic temperatures and plug passages and plate out on cold surfaces. Atmospheric air also contains several hydrocarbon impurities that must not be allowed to come in contact with liquid oxygen. Typically, hydrocarbon impurities enter the cold box, and accumulate in the liquid oxygen sump in the reboiler of the low-pressure column. The air separation process also concentrates the levels of these impurities, such that their concentrations in the low-pressure column are several times greater than their concentrations in the PPU product.
When the solubility and flammability limits of these compounds in liquid oxygen are exceeded, these compounds may be combusted, leading to small explosions and in extreme cases, to the combustion of the distillation column internals. The accumulation of hydrocarbon impurities can also take place inside the passages of heat exchangers and reboilers causing the same problems as seen in the distillation column. This accumulation in passages of heat exchangers is aggravated by the presence of plugging/plating components. Plated N2O and CO2 may create pockets of stagnant liquid oxygen in which hydrocarbons can accumulate and concentrate beyond flammability limits.
It is clear that plugging components and hydrocarbons pose a combined threat to ASU safety, in addition to their individual deleterious effects. These observations highlight the need to remove in the PPU process not only N2O but hydrocarbons as well. The present invention is directed to such a solution that aims, in particular, to a simultaneous, highly effective pre-purification of air in a thermal swing adsorption process.
The present invention provides for a novel process for the removal of impurities from a gas stream by thermal swing adsorption. The process comprises passing a gas stream containing impurities through a first adsorbent that is capable of removing water from the gas stream, then passing the stream through a second adsorbent layer which is capable of removing carbon dioxide from the gas. Lastly, the gas stream is passed through a third layer capable of removing hydrocarbons and oxides of nitrogen from the gas.
The third adsorbent is a composite adsorbent that contains at least one adsorbent that will adsorb preferentially oxides of nitrogen and a series of hydrocarbons, ie., xe2x80x9cN2O removal adsorbentxe2x80x9d, and at least one adsorbent that will adsorb preferentially those hydrocarbons that were adsorbed by the first adsorbent to lesser an extent, i.e., xe2x80x9chydrocarbon removal adsorbentxe2x80x9d. Preferably, the xe2x80x9cN2O removal adsorbentxe2x80x9d is a CaLSX, CaMSX or CaX type zeolite and the xe2x80x9chydrocarbon removal adsorbentxe2x80x9d is a CaA type zeolite.
A composite adsorbent as to this invention is understood as a physical mixture of primary particles (micro-particles) of at least two types of adsorbents, e.g., zeolites mentioned, that stem from separate syntheses, and shaped by extruding or beading or another known method into macro-particles of the final type, using a binder material.
In a preferred embodiment of the present invention, there is provided a process comprising passing a gas stream containing impurities through a first adsorbent which is capable of substantially removing carbon dioxide and moisture from the gas stream and then passing the gas stream through a second adsorbent layer which is capable of removing hydrocarbons and oxides of nitrogen from the gas. In this embodiment, the first layer is a zeolite and the second layer is the composite adsorbent.
In a different embodiment of the present invention, there is provided a process comprising passing a gas stream containing impurities through a first adsorbent which is capable of substantially removing moisture from the gas stream and then passing the gas stream through a second adsorbent layer which is capable of removing carbon dioxide, hydrocarbons and oxides of nitrogen from the gas. In this embodiment, the first layer is activated alumina and the second layer is the composite adsorbent.
In another preferred embodiment, there is provided a process comprising passing a gas stream containing impurities through the composite adsorbent which will substantially remove water, carbon dioxide and hydrocarbons and oxides of nitrogen from the gas. In this embodiment, the composite adsorbent comprises a first adsorbent and a second adsorbent.
The advantage of the present invention resides in its relative simplicity compared to using a fourth adsorbent layer. The use of the composite adsorbent will convert the temperature swing adsorption pre-purification unit into a unit which will not only remove water and carbon dioxide from gas streams prior to entry into the cryogenic air separation unit but will also remove the undesirable nitrogen oxides, especially dinitrogen oxide, and saturated and non-saturated hydrocarbons from the gas stream. This leads to a gas stream entering the cryogenic distillation unit that is substantially free of trace impurities. The resultant highly purified air stream entering the cryogenic air separation unit results in a much safer plant operation and enables the air separation unit to give products that have an even higher purity than previously attained.